There are 20 painters in all who worked on this job.
Given:
interior painters = 15 painters
percentage of interior painter = 75% of painting crew
15 is a part of a whole ; 75% is the equivalent of the part in a whole.
To get the whole which is the total number of painters in the painting crew, we divide the part by its percentage.
15 / 75% = 20 painters in a painting crew
To check:
20 x 75% = 15
15/20 = 0.75 x 100% = 75%